This invention relates to an electron emission system and, more particularly, to such a system that includes a high-brightness electron source of the field-emitter type.
In a variety of applications of practical importance, the need exists for a stable high-brightness source of electrons. These applications include, for example, electron beam exposure systems of the type utilized in the fabrication of integrated circuits. Early versions of these systems employed thermionic thoriated-tungsten electron sources. More recently, as the need arose for still-higher-brightness electron sources, thermionic emitters made of lanthanum hexaboride (LaB.sub.6) have been proposed. Typical emitters made of LaB.sub.6 are generally at least about four times brighter than thoriated-tungsten sources.
But, in the course of designing a new generation of very-high-speed electron beam exposure systems, it became apparent that even LaB.sub.6 sources would not satisfy the electron emission requirements of such equipment. What was needed was a source capable of a brightness about one hundred times that of a thoriated-tungsten source. And, although it was known that very-high-brightness electron sources of the field-emitter type were available, the operating characteristics of such known sources did not meet the stringent emission stability requirements of a highly reliable electron beam exposure system suitable for commercial use.
Accordingly, efforts have been directed at trying to improve the operation of a field-emitter electron source and of an overall system including such a source. It was recognized that such efforts, if successful, could provide an important basis for a successful commercial design of electron emission equipment such as a very-high-speed electron beam exposure system. In turn, the availability of such a system would lower the cost of fabricating very-large-scale integrated circuit devices.